Author Topic: 40v from solar panel into 12v battery  (Read 3015 times)

0 Members and 1 Guest are viewing this topic.

AbyssUnderground

  • Sr. Member
  • ****
  • Posts: 461
40v from solar panel into 12v battery
« on: February 24, 2006, 07:34:49 PM »
Would it be ok to use two 12v (21v open) 1.5w solar panels in series to charge a 17.2Ah SLA battery? Ive heard batterys drag the voltage down when its too high. Would this result in a higher charger current too?
« Last Edit: February 24, 2006, 07:34:49 PM by (unknown) »

Ungrounded Lightning Rod

  • SuperHero Member
  • ******
  • Posts: 2865
Re: 40v from solar panel into 12v battery
« Reply #1 on: February 24, 2006, 01:06:58 PM »
It would work just fine.


But (assuming it's a 12V battery) you'll get twice the charging current if you reconnect the panels in parallel.


With the seriesed panels you'll get the same charging current as if you had fewer panels in series or lower voltage panels (down to where you have only enough extra open-circuit volts above your battery voltage to cover resistive losses and panel voltage reduction from high panel temperature).


Solar panels are essentially current limited devices.  As long as your panel voltage is sufficiently above the battery voltage that it is the panel's current output that limits the charging current (rather than voltage drop in the panels' wiring resistance) the charging current will be the same no matter how many panels are in series.  The number of electrons you get is limited by the number of photons striking the least illuminated (parallel set of) cell(s).


The extra energy is dissipated as heat in the panel.  It's not an issue, since completely shorting the panel or array causes its total heating to be the same as leaving it out in the sun unconnected to anything.  Pulling any power at all reduces the heating by the amount of energy you pulled.  Solar regulators generally prevent overcharging by shorting the panel out.  The anti-backflow diode keeps the battery from discharging into the short the regulator applies.


Does anyone know if a 17.5 Ah SLA can recombine hydrogen and oxygen fast enough to handle 150 milliamps of overcharge trickle current?  If not you'll need some way to  protect them from overcharging.


(Fortunately there's little enough power to dissipate that you could just hang a 2W zener at your float voltage across your battery as a dump load - or a couple of 1W zeners in series at half the float voltage.  Double the power rating if you decide to parallel the panels.)

« Last Edit: February 24, 2006, 01:06:58 PM by Ungrounded Lightning Rod »

AbyssUnderground

  • Sr. Member
  • ****
  • Posts: 461
Re: 40v from solar panel into 12v battery
« Reply #2 on: February 25, 2006, 02:54:26 AM »
Thanks for the reply. Im sure it can handle 150mA as the charger is 500mA and it happilty sits there for days at full charge. All I see is a rise in voltage to 14.5v. Im sure it will be fine.


Thanks for all the advice and information too. The members of this board are priceless :-)

« Last Edit: February 25, 2006, 02:54:26 AM by AbyssUnderground »

PaulJ

  • Full Member
  • ***
  • Posts: 111
Re: 40v from solar panel into 12v battery
« Reply #3 on: February 25, 2006, 03:13:17 AM »
   "Does anyone know if a 17.5Ah SLA can recombine hydrogen and oxygen fast enough to handle 150 milliamps of overcharge trickle current?"


   At the end of a good day my 400Ah sealed battery bank floats on about 600mA. A 17.5 Ah SLA will require some sort of regulation if left unattended on a 150mA charge.


   Paul.

« Last Edit: February 25, 2006, 03:13:17 AM by PaulJ »

PaulJ

  • Full Member
  • ***
  • Posts: 111
Re: 40v from solar panel into 12v battery
« Reply #4 on: February 25, 2006, 03:22:59 AM »
  I must have replied at the same time as you did. Are you sure your charger doesn't have some sort of regulator? 500mA would cook a 17.5Ah sla fairly quickly in my opinion. Check the current from the charger when the battery is at 14.5V, I'll bet it's nowhere near 500mA.


   Paul.

« Last Edit: February 25, 2006, 03:22:59 AM by PaulJ »

AbyssUnderground

  • Sr. Member
  • ****
  • Posts: 461
Re: 40v from solar panel into 12v battery
« Reply #5 on: February 25, 2006, 10:22:11 AM »
Actually this is not true. I have a 10A 13.8v power supply. I leave it permanently connected to a 7Ah lead acid as a backup for my amateur radio equipment. Its very rarely run down. I worried about this to begin with. I connected it up fully charged and it only took around 9mA from the power supply. Not sure if this is the same principal with solar panels but the battery only seemed to take what it wanted. It wasnt forced power it didnt want.
« Last Edit: February 25, 2006, 10:22:11 AM by AbyssUnderground »

ghurd

  • Super Hero Member Plus
  • *******
  • Posts: 8059
Re: 40v from solar panel into 12v battery
« Reply #6 on: February 25, 2006, 10:31:11 AM »
A 13.8V supply may be regulated to 13.8V.  The solar panels will make peak power at around 17V.  

I personally wouldn't be overly concerned with 1 panel, but you may want to have a look at an IC, PB137ACV. A TO-220 (~ish) and 2 caps.  It won't take 2 panels in series, but it will take them in parallel.  I don't use the 2 caps, just use the IC, no problems here.

G-
« Last Edit: February 25, 2006, 10:31:11 AM by ghurd »
www.ghurd.info<<<-----Information on my Controller

AbyssUnderground

  • Sr. Member
  • ****
  • Posts: 461
Re: 40v from solar panel into 12v battery
« Reply #7 on: March 02, 2006, 12:04:31 PM »
After a sunny morning here I was able to test the two panels in series. They kept the battery voltage at 13.64v (fully charged). When I disconnected them the voltage dropped to 13.3v (the levelled off voltage for it fully charged).


It works very well and charges slowly in low light. Im yet to get a decent sunny day to try them in parallel.

« Last Edit: March 02, 2006, 12:04:31 PM by AbyssUnderground »